High power bipolar pulse generators

ABSTRACT

A bipolar pulse generator is implemented in a simple structure while providing a high efficiency design having a relatively low total size, while still allowing access by fibers used to control a photoconductive switch that activates the generator. The bipolar pulse generator includes a stacked Blumlein generator structure with an additional transmission line connected to a load at its near end and short-circuited at its distal end. An extra transmission line is positioned between the Blumlein generator&#39;s structure and the load provides specified limited gap between positive and negative sub-pulses. The bipolar pulse generator further includes a bended Blumlein generator structure, in which an existing intrinsic “stray” transmission line is used to provide the bipolar pulse. Still further, bipolar pulse generator includes stepped transmission lines, with additional switches positioned between steps, which are charged by different voltages.

REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims the benefit of priorityto prior U.S. patent application Ser. No. 12/404,061 by inventor SimonY. London, entitled “HIGH POWER BIPOLAR PULSE GENERATORS” filed on Mar.13, 2009.

BACKGROUND

The present invention relates in general to pulse generators. Morespecifically, the present invention relates to bipolar pulse generatorsthat provide a high power/energy pulse on a load.

Recent trends in the development of pulse power microwave sources for avariety of applications have been directed to increasing power,efficiency and energy on the load. Transmission line pulse generatorswith different kinds of fast switches, including light activatedphotoconductors, can achieve some of the best results in generating highpower short duration pulses. For a given limited charging voltage oftransmission lines defined by high-current switches, high powered andhigh energy density transmission lines imply low characteristicimpedances. This low range of characteristic impedances, however,frequently causes problems for coupling with typically used loadimpedances, 50 ohm or higher, for example, radiating impedances, whichintroduces a problem with efficient high ratio impedance transformation.

Bipolar pulse generators very often have significant advantages comparedto unipolar pulse generators, with just one example being UWB radiation.Further, there are many potential applications of bipolar pulsegenerators, for example in industry, physics and medicine, where veryoften bipolar pulse generators with time separation between positive andnegative sub-pulses are preferable or required. Today, however, thereare only various types of high power and high energy unipolar pulsegenerators (Marx generator and stacked Blumlein generator in variousmodifications).

For example, a high energy Marx generator with coaxial cable to providerectangular unipolar pulse is known and described in “A PFN MarxGenerator Based on High-Voltage Transmission Lines”, by S. M. Turnbullet al., presented in Meas. Sci. Technol. 11 (2000) N51-N55. Further, astacked Blumlein generator with a single switch has been proposed inU.S. Pat. No. 2,769,101 issued to R. D. Drosd. This type of generatorhas been designed and presented in various publications including, forexample, “Modeling of Wound Coaxial Blumlein Pulsers”, by Jose O. Rossiet al:, published in IEEE Transactions on Plasma Science”, Vol. 34, No.5, October 2006, “Design of a 150 kV, 300 A, 100 Hz Blumlein CoaxialPulser for Long Pulse Operation”, presented in IEEE Transactions onPlasma Science”, Vol. 30, No. 5, October 2002. Still further, somemodifications of stacked (cascade) Blumlein generators are presented in“A Combined High-Voltage, High-Energy Pulse Generator”, by S. J.MacGregor et al., published in “Meas. Sci. Technol” 5 (1994), pp.1580-1582, and “A Novel HV Double Pulse Modulator”, published in “Meas.Sci. Technol” 5 (1994), pp. 1407-1408. Finally, another type ofhigh-power generator, namely, a “Multi-Stage Blumlein” is proposed by J.Yampolsky in US Patent Application 2005/0174715 A1, 2005. The content ofeach of the above-reference documents is incorporated herein byreference. All of the above-referenced generators produce only aunipolar pulse and do not provide voltage (impedance) transformation,with the exception of the proposed multi-stage Blumlein disclosed in USPatent Application 2005/0174715A1, which provides moderatetransformation but requires a substantial number of switching devices.

A transmission line “High-Voltage Pulses Generator” has also beendescribed in SU Patent 1098502 A1 issued to Bosamykin V. S. et al, 1996,which provides bipolar pulse by a single switch. However, thepower/energy on load is much less compared to that provided by theabove-mentioned unipolar generators. In addition, impedancetransformation in the device is low.

The applicant has also previously described a transmission line in U.S.Patent Application 2007/0165839 A1 entitled “Bipolar Pulse GeneratorsWith Voltage Multiplication”, which provides a device with a singleswitch with all of the required voltage/impedance transformation.However, in a stacked configuration with several switches, the energyprovided by this type of generator is less compared to the abovementioned Blumlein-based stacked unipolar generators with less number ofswitches.

Accordingly, there remains a need for a bipolar pulse generator solutionbased on voltage charged transmission lines, which provides high powerand high energy. Further, there remains a need for high power/energybipolar pulse generator, which can provide voltage/impedancetransformation. Still further, there remains a need for a highpower/energy bipolar pulse generator with pulse separation betweenpositive and negative sub-pulses.

It would be desirable to provide a bipolar pulse generator that couldmeet all of the above needs while being implemented in a simplestructure, preferably with a single switch, and preferably in a highefficiency design that has a relatively low total size, while stillallowing simple access by fibers to a closing photoconductive switchthat actuates the bipolar pulse generator.

SUMMARY OF THE INVENTION

The present invention provides a bipolar pulse generator that can beimplemented in a simple structure while providing a high efficiencydesign having a relatively low total size and still allowing access byfibers used to control a photoconductive switch that activates thegenerator.

In a preferred embodiment of the invention, a bipolar pulse generatorincludes a stacked Blumlein generator structure with an additionaltransmission line connected to a load at its near end andshort-circuited at its distal end. An extra transmission line ispositioned between the Blumlein generator's structure and the loadprovides specified limited gap between positive and negative sub-pulses.

According to a further preferred embodiment of the present invention,the bipolar pulse generator further includes a bended Blumlein generatorstructure, in which an existing intrinsic “stray” transmission line isused to provide the bipolar pulse.

According to a still another embodiment of the present invention, thebipolar pulse generator consists of stepped transmission line withadditional switches positioned between steps, which are charged bydifferent voltages.

The bipolar pulse generator according to the invention generates highpower/energy pulses in a compact design with access to fibers foractivating photoconductor switches. Bipolar pulse generators accordingto the invention are useful for HPM generation, in particle acceleratorsand in other high voltage physical, industrial, medical and testinstruments.

Other features, uses, advantages, embodiments, etc. of the inventionwill become apparent to those skilled in the art from the followingdetailed description of the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to certain preferredembodiments thereof along with the accompanying figures, wherein:

FIG. 1 depicts a schematic of classic n-stacked Blumlein pulse generatoraccording to the prior art;

FIG. 2 depicts a schematic of double stacked bipolar pulse generatoraccording to the prior art;

FIG. 3 depicts a schematic of two-step bipolar pulse generator as anexpansion of the generator in FIG. 2 for impedance transformationaccording to the prior art;

FIG. 4 depicts a schematic of double Blumlein pulse generator accordingto the prior art;

FIG. 5 depicts a schematic of two series connected double Blumlein pulsegenerators according to the prior art;

FIG. 6 depicts a schematic of bipolar pulse generator with two switchespositioned in first two successive steps according to the prior art;

FIG. 7 a depicts a schematic of transmission line Marx-based bipolarpulse generator according to an embodiment of the present invention;

FIG. 7 b depicts a pulse shape on the load of generator shown on FIG. 7a;

FIG. 8 a depicts a schematic of an n-stacked Blumlein based bipolarpulse generator according to an embodiment of the present invention;

FIG. 8 b depicts a pulse shape on the load of generator shown on FIG. 8a;

FIG. 9 depicts a schematic of a three-step, two-stacked Blumlein basedbipolar pulse generator according to an embodiment of the presentinvention;

FIG. 10 depicts a table of normalized element's values of an N-step,two-stacked Blumlein-based bipolar pulse generators according to anembodiment of the present invention;

FIG. 11 a depicts a schematic of a double Blumlein-based bipolar pulsegenerator according to an embodiment of the present invention;

FIG. 11 b depicts a pulse form on the load for generator according toFIG. 11 a;

FIG. 12 a depicts a schematic of a double Blumlein-based bipolar pulsegenerator with their intrinsic transmission lines according to anembodiment of the present invention;

FIG. 12 b depicts a pulse form on the load for generator according toFIG. 12 a;

FIG. 13 a depicts a schematic of two series connected double Blumleinbased bipolar pulse generators with their intrinsic transmission linesaccording to an embodiment of the present invention;

FIG. 13 b depicts a schematic of two series connected double Blumleinbased bipolar pulse generators in case of neglecting of intrinsictransmission lines according to an embodiment of the present invention;

FIG. 14 depicts a schematic of a series connected N double Blumleinbased bipolar pulse generators with their intrinsic transmission linesaccording to an embodiment of the present invention;

FIG. 15 a depicts a schematic of double single-stage bipolar pulsegenerator with their intrinsic transmission lines that provides abipolar pulse without a gap between sub-pulses according to anembodiment of the present invention;

FIG. 15 b illustrates the pulse form on load for generator according toFIG. 15 a;

FIG. 16 depicts a schematic of series connected two single-stage doublebipolar pulse generators with their intrinsic transmission linesaccording to an embodiment of the present invention;

FIG. 17 depicts a schematic of series connected two single-stage, doublebipolar pulse generators without (neglecting) their intrinsictransmission lines according to an embodiment of the present invention;

FIG. 18 a depicts a schematic of two-step, double bipolar pulsegenerators with their intrinsic transmission lines according to anembodiment of the present invention;

FIG. 18 b illustrates the pulse form on the load provided by thegenerator according to FIG. 18 a;

FIG. 19 depicts a schematic of three-step bipolar pulse generator withtwo switches in first two successive steps and with the gap betweensub-pulses equal to the length of sub-pulse as an embodiment of thepresent invention;

FIG. 20 illustrate the table of multi-steps bipolar pulse generatorswith two switches in first two successive steps and with the gap betweensub-pulses equal to the length of sub-pulse according to an embodimentof the present invention;

FIG. 21 a depicts a totally folded design of N-step bipolar pulsegenerator with two switches in first two successive steps and with thegap between sub-pulses equal to the length of sub-pulse according to anembodiment of the present invention; and

FIG. 21 b depicts a partial-folded design of N-step bipolar pulsegenerator with two switches in first two successive steps and with thegap between sub-pulses equal to the length of sub-pulse according to anembodiment of the present invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts a well-known stacked Blumlein pulse Generator thatprovides a high-energy unipolar pulse on a matched load. FIG. 2 depictsa schematic of a prior art double stacked bipolar pulse generator of thetype described in US Patent Application 2007/0165839 A1. The storedenergy and the energy on the load, however, is 75% of the energyprovided by double stacked (n=2) unipolar generator presented on FIG. 1.FIG. 3 depicts a schematic of a prior art bipolar pulse generator, whichis an extended type of generator shown in FIG. 2 with and additionalimpedance transformation step. FIG. 4 depicts a schematic of a prior artunipolar pulse generator, which is a double Blumlein pulse generator(with interconnected open-circuited charged transmission lines)presented by S. J. MacGregor et al. discussed above. FIG. 5 depicts aschematic of a prior art unipolar pulse generator, which is two seriesconnected double Blumlein pulse generators (with interconnectedopen-circuited charged transmission lines) presented by S. J. MacGregoret al. discussed above. FIG. 6 depicts a schematic of a prior artbipolar pulse generator with two switches in first two successive stepspresented in US 2007/0165839 A1. The power/energy of the generatingpulse is not a maximum that could be achieved in similar structure withtwo switches positioned in first two successive steps. The inventionwill be described in part with reference to prior art structures such asthose discussed above.

FIG. 7 a is a schematic of Marx-based transmission line bipolar pulsegenerator according to an embodiment of the present invention. Thegenerator may consist of any number (n) of identically chargedtransmission lines 10. Each transmission line 10 is connected to acorresponding individual switch 11 and to a corresponding individualcharging element 12 (resistor R or inductance L). Instead of typicaldirect connection to the load 15, the load 15 is connected through anadditional transmission line 13 of a specified length. In addition, atransmission line 14 is connected to the load 15 at its near end and isshort-circuited at its distal end. The electrical length of thetransmission line 14 is equal to the sum of length of each charged line10 and the length of line 13. The described arrangement provides aspecified gap between positive and negative sub-pulses that is equaldouble the transit time of transmission line 13.

In operation, all of the charged transmission lines 10 are charged bytheir individual charging element 12. Once all the charged transmissionlines 10 are fully charged, all of the switches 11 are closed at thesame moment of time, thereby causing the charged transmission lines 10to operate as n series connected generators. As a result, a bipolarpulse with a predicted time space or gap between positive and negativesub-pulses is realized on the load 15 as is illustrated in FIG. 7 b.

FIG. 8 a is a schematic of a stacked Blumlein-based bipolar pulsegenerator according to an embodiment of the present invention. Thegenerator consists of a charging structure 30 with any number n ofidentically first charged transmission lines 31 with switches at theirnear ends, and n oppositely charged second transmission lines 32 withthe same length and characteristic impedances as for the firsttransmission lines 31. The output of this stacked Blumlein structure 30is connected to the near end of an additional non-charged transmissionline 33 with a specified electrical length t1 and characteristicimpedance equal to 2nZ0, where Z0 is a characteristic impedance of eachcharged first transmission lines 31 and the second transmission lines32. The load 34 is connected to the distal end of the transmission line33. In addition, another transmission line 35 is provided, which isconnected to the load 34 at its near end and is short-circuited at itsdistal end. The load impedance is equal to nZ0, while the characteristicimpedance of transmission line 35 is the same as for transmission line33. The electrical length of the transmission line 35 is equal 2 t+t1,where t is electrical length of each of the first transmission lines 31and the second transmission lines 32.

During operation, all the transmission lines 31 and the transmissionlines 32 are charged by a voltage supply V0. All of the n switches arethen closed simultaneously and a wave propagation process occurs.Identical waves propagate on all of the charged transmission lines 31and the same is true for all of the charged transmission lines 32. Theresulting pulse on the load is illustrated on FIG. 8 b and minimumseparation between sub-pulses is equal 2 t.

Referring to FIG. 9, a schematic of a three-step two stacked Blumleinbased bipolar pulse generator according to an embodiment of theinvention is illustrated. The generator starts from a generatoraccording to FIG. 8 a for particular case n=2 (transmission lines 41 andtransmission lines 42) and t1=0. Extra step transmission lines 43 and 44are provided as well as a transmission line 45 with specificcharacteristic impedances, obtained by impedance transformationprocedure applied to initial circuit (FIG. 8 a for n=2 and t1=0). Thisprovides additional impedance/voltage transformation. The chargedstructure 40 of this generator consists of transmission lines 41, 42,43, 44 and 45. Load 46 is positioned between charged transmission linestructure 40 and a transmission line 47 that is connected to the load 46and short-circuited at its distal end. The bipolar pulse is initiated bysimultaneously closing two switches 48.

FIG. 10 is a table of normalized characteristic impedances oftransmission lines as well as load impedances for odd numbers of steps1, 3, 5 . . . 19. The table illustrates the rate of increasing impedancetransformation by increasing the number of steps. The pulse form isindependent on the number of steps. Only the magnitude of pulse isincreased from step to step.

FIG. 11 a is a schematic of a double Blumlein based bipolar pulsegenerator according to an embodiment of the present invention. Thisgenerator consists of a known double Blumlein unipolar pulse generatorstructure (transmission lines 60, 61, 62 and switch 63) with additionaltransmission lines 64 and 65. A transmission line 64 with time delay t1is connected between the output of the double Blumlein unipolar pulsegenerator structure and a load 66. Transmission line 65 is connected tothe load 66 at its near end and is short-circuited at its distal end. Acharacteristic impedance of the transmission lines 64, 65 is twice theimpedance of the load 66 and four times more then the impedance of eachof the transmission lines 60, 61 or 62. The electrical length of thetransmission line 65 is twice the length of transmission lines 60 or 61and is equal to the length of transmission line 62. Line 62 could alsobe separated in the middle by two identical length transmission lineswithout any change in operation and in pulse form on the load 66.

Ideal operation of this generator is similar to that for generatoraccording to FIG. 8 a when the number of switches equal two (n=2). Theresulting pulse form on the load 66 is illustrated on FIG. 11 b. Theideal operation of the generator according to FIG. 11 a assumes thatthere are no inductances by outer conductors, or more correctly, notransmission lines associated by outer conductors of transmission lines60 and 61, i.e. between nodes a1 and a2, as well as between nodes a1 andb1 by outer conductors of transmission line 62. However, between thesenodes, there always exists intrinsic (stray) transmission linesshort-circuited at their distant ends in practice. These transmissionlines with specific characteristic impedances and electrical lengthcould be used instead of transmission line 65 (or in addition totransmission line 65 with increased characteristic impedance) to providea bipolar pulse. However, it is valid only for the case t1=0 andillustrated on FIG. 12 a.

Referring to FIG. 12 a, which is a schematic of double Blumlein-basedBipolar Pulse generator as an embodiment of the present invention. Thisgenerator consists of known double Blumlein unipolar pulse generatorstructure with transmission lines 70, 71, 72 and switch 73. Thisstructure is similar to the structure with transmission lines 60, 61, 62and switch 63 of FIG. 11 a. However, instead of using transmission line65 with time delay 2 t=0), which is short-circuited at its distal end,there are two intrinsic transmission lines 74 and 75 formed by outerconductors of transmission lines 70, 71 and by a folded outer conductorof transmission line 72. Transmission lines 74 and 75 are connected inseries relative to load 76 with resulting characteristic impedance equalto 4Z and operate in the same manner as the transmission line 64 of FIG.11 a. The electrical length of each of these lines should also be equalto 2 t.

It should be noted that combined design of FIG. 11 a for t1=0 and FIG.12 a is also possible. Accordingly, in addition to the two intrinsictransmission lines 74 and 75 of FIG. 12 a (with impedances more then 2Zeach), the transmission line 65 of FIG. 11 a with characteristicimpedance more then 4Z could be used.

FIG. 13 a is a schematic of a series connected two double Blumlein basedbipolar pulse generators according to an embodiment of the presentinvention. This generator consists of a double Blumlein-based bipolarpulse generator's structure (transmission lines 80, 81, 84, 86 and 88),which is the same as the generator on FIG. 12 a and it is connected inseries with exactly the same generator's structure (transmission lines82, 83, 85, 87 and 89). Both switches 91 and 92 should be closedsimultaneously. These two switches could be replaced by a single switchor by any number of simultaneously closed switches. It should be notedthat intrinsic transmission lines 86, 87, 88 and 89 (if they areneglected) could be replaced by a single (or two) transmission line(s)as shown in FIG. 13 b (transmission line 95 or transmission lines 94 and95 if t1>0). In the case of two lines, transmission line 94 with timedelay t1 provides an additional 2 t 1 separation between sub-pulses,i.e. total time separation 2(t+t1) could be implemented. Any negativeeffect by the intrinsic transmission lines can be minimized by properdesign.

FIG. 14 is a schematic of series connected N double Blumlein basedbipolar pulse generator according to an embodiment of the presentinvention. This generator consists of double Blumlein-based bipolarpulse generator structure (transmission lines 100, 101, 106, 109 and112), which is the same as generator in FIG. 12 a and is connected inseries with exactly the same generator structure (transmission lines102, 103, 107, 110 and 113). This second double Blumlein-based bipolarpulse generator structure is also connected in series with the next thesame generator structure and finally with the last N-th generatorstructure (lines 104, 105, 108, 111 and 114). All simultaneously closedN switches 116, 117 . . . 118 could be replaced by a single switch or byany number of switches. The load 115 is a result of series connectionmatched loads of individual double Blumlein-based Bipolar PulseGenerators with their summarized impedance 2NZ.

By analogy with the generators shown in FIG. 13 a and FIG. 13 b, all ofthe intrinsic transmission lines 109, 110, . . . 111 and 112, 113, . . .114 could be replaced by a single line connected to the load 115 at itsnear end and short-circuited at its distal end, which is similar totransmission line 95 (FIG. 13 b) when t1=0. For an extended gap betweensub-pulses (t1>0), an additional transmission line as transmission line94 in FIG. 13 b should be used. If various combined solutions forshort-circuited at their distal end transmission line and intrinsiclines as discussed above with respect to FIGS. 11 a, 12 a and FIG. 13 a,could be used depending on specific designs issues. It should be notedfor all generators shown in FIGS. 11 a, 12 a, 13 a and 14, variouspositions of ground connections can be used including single ground orno connections to ground.

FIG. 15 a is a schematic of double single-stage bipolar pulse generatoraccording to an embodiment of the present invention. By analogy with thedouble Blumlein-based bipolar pulse generators according to FIG. 12 a,this generator is obtained by interconnection of two bipolar pulsegenerators. However, in this case there is no gap between positive andnegative sub-pulses. Two switched transmission lines 120 and 121 arecombined with a single switch 126 and a single non-switched transmissionline 122 is provided with double length 2 t. Two intrinsic equal-lengthtransmission lines 123 and 124 with impedances Z2 and Z1, respectively,are provided. For this generator, no connections to ground or differentground connections, including shown on FIG. 15 a, could be used.Independent on connections to ground, different combinations ofcharacteristic impedances Z1 and Z2 without deterioration of thegenerating pulse are acceptable. Assuming Z=1 (normalization) some ofthese combinations are presented in Table 1 below.

TABLE 1 Z1 (Z2) 8 7 6 5 4 3 2.5 2 Z2 (Z1) 8 9.2 11 14 20 38 74 ∞

FIG. 15 b illustrates pulse shape on the load 125 independent on valuesZ1 and Z2. In the case when Z1 and Z2 are very high (Z1, Z2>>Z, i.e.intrinsic lines 123 and 124 are neglected), the bipolar pulse accordingto FIG. 15 b could be achieved by using a transmission line connected tothe load 125 at its near end and short-circuited at its distal end, aswas shown on FIG. 11 a and FIG. 13 b. To provide separation betweensub-pulses an extra transmission line like transmission line 64 in FIG.11 a or transmission line 94 in FIG. 13 b should be used.

Referring to FIG. 16, which is a schematic of generator, which consistsof two series connected double single-stage bipolar pulse generatorsshown on FIG. 15 a as an embodiment of the present invention. Thisgenerator consists of double single-stage bipolar pulse generatorstructure (lines 130, 131, 134, 136 and 138), which is the same asgenerator structure on FIG. 15 a and it is connected in series withexactly the same generator structure (transmission lines 132, 133, 135,137 and 139). Both switches 141 and 142 should be closed simultaneously.These two switches could be replaced by a single switch or by any numberof simultaneously closed switches.

In the case when impedances Z1 and Z2 of intrinsic transmission lines136, 137, 138 and 139 are much more compared to Z (and 3Z) the structureFIG. 13 b is valid with specific values of load impedance 140, whichshould be equal to 32Z/3 as shown on FIG. 17. Characteristic impedanceof transmission line 143 should be equal to 32Z and their electricallength is equal t. Separation between sub-pulses will be also equalzero.

FIG. 18 a is a schematic of bipolar pulse generator as an embodiment ofthe present invention. This generator consists of two-step, doublesingle-stage bipolar pulse generator structure with intrinsictransmission lines that provides bipolar pulse. This generator operates,in principle, as generator according to FIG. 7 a in mentioned above USPatent Application 2007/0165839 A1. First step with transmission lines150 and 152 is connected to the second step with transmission lines 151and 153, while transmission line 154 with double electrical length 2 tplay the same role as two transmission lines 340 in mentioned abovegenerator, according to FIG. 7 a. Intrinsic lines 155 and 156 connectedin series play the same role as transmission line 385 in mentioned abovegenerator according to FIG. 7 a. FIG. 18 b illustrates the pulse form onthe load provided by generator according to FIG. 18 a.

FIG. 19 is a schematic of bipolar pulse generator with two switches infirst two successive steps as an embodiment of the present invention.This generator provides bipolar pulse with gap between sub-pulses equalto the length of sub-pulse. The charging voltage 2V of second and thirdsteps is twice the charging voltage V of the first step. In this casevoltages on both switches 164 and 165 are identical and equal V. Mostlybecause of characteristic impedance of the second step line 161 is abouttwice the characteristic impedance of the first step line 160 the totalenergy stored in this generator, i.e. in lines 160, 161, 162 and 163 ismuch higher compared to single-switch stepped-line generator, as well astwo-switches generator according to FIG. 20 in US Patent Application2007/0165839 A1.

During operation if switch 164 is turned ON (closed) at time to, thesecond switch 165 should be turned ON (closed) at time t0+t, i.e., attime slightly less than t after t0 to prevent overvoltage on switch 165.However, in the case of switch 165 is a spark-gap it will be turned ONautomatically due to overvoltage. The impedance transformation (ZL/Z) asa ratio of load impedance 167 to the lowest impedance Z of the firststep 160 and ratio of inductive stub 166 to the load impedance will beincreased by increasing the number of steps without deterioration thepulse shape. FIG. 20 shows a table of normalized element's values forN-step (N=3, 5, 7 . . . 23) bipolar pulse generator according to FIG. 19and illustrate the increasing transformation ratios.

FIGS. 21 a and 21 b illustrates folded and partly folded designs ofbipolar pulse generator according to FIG. 19, as an embodiment of thepresent invention. Both switches 164 and 165 are positioned outsidestructure that is preferable for practical implementation.

The invention has been described with reference to certain preferredembodiments thereof. It will be understood by those skilled in the artthat modifications and variations are possible with the scope of theappended claims.

1. A bipolar pulse generator comprising: a double Blumlein unipolarpulse generator structure including transmission lines; a firsttransmission line with a time delay connected between the output of thedouble Blumlein unipolar pulse generator structure and a load; a secondtransmission line connected to the load at its near end and shortcircuited at its distal end.
 2. A bipolar pulse generator as claimed inclaim 1, wherein a characteristic impedance of each of the first andsecond transmission lines is twice the impedance of the load and fourtimes the impedance of the transmission lines of the pulse generatorstructure.
 3. A bipolar pulse generator as claimed in claim 1, whereinthe second transmission line comprises two intrinsic transmission linesformed by outer conductors of first and second transmission lines of thepulse generator structure and by a folded outer conductor of a thirdtransmission line of the pulse generator structure, wherein the twointrinsic transmission lines are connected in series relative to load.4. A bipolar pulse generator as claimed in claim 1, further comprisingat least one additional double Blumlein unipolar pulse generatorstructure connected in series with the double Blumlein unipolar pulsegenerator.
 5. The bipolar pulse generator of claim 4, wherein theswitches of all the series connected double Blumlein unipolar pulsegenerators are configured to close simultaneously.
 6. The bipolar pulsegenerator of claim 1, wherein the characteristic impedance of the secondtransmission line is greater than twice the impedance of the load. 7.The bipolar pulse generator of claim 1, wherein the electrical length ofthe second transmission line is greater than the electrical length ofthe first transmission line.
 8. The bipolar pulse generator of claim 1,wherein all the transmission lines have the same voltage charge.